It is well known to conduct chemical sensitization using a labile compound such as sulfur compounds, selenium compounds, tellurium compounds, gold compounds, platinum compounds or palladium compounds, or a combination of these compounds for producing a high-sensitive silver halide photographic emulsion and a high-sensitive photographic material.
U.S. Pat. No. 3,772,031 describes a method for obtaining a high-sensitive emulsion by uniformly doping an ion of a chalcogen group inside a grain and JP-B-46-4553 (the term "JP-B" as used herein means an "examined Japanese patent publication") describes a method for obtaining a high-sensitive photographic emulsion by unstabilizing a non-labile selenium compound with a reducing agent and effectively controlling the chemical sensitization of grain.
As a means for achieving higher sensitivity, U.S. Pat. Nos. 5,166,045 and 5,164,292 describe a method for obtaining a high-sensitive photographic emulsion by doping the above-described non-labile selenium compound in an amount, in terms of selenium added, of from 0.05.times.10.sup.-8 to 0.62.times.10.sup.-8 mol per the unit surface area (1 m.sup.2) of the emulsion grain after, in the former, from 65 to 90% (U.S. Pat. No. 5,166,045) or 50% or more (U.S. Pat. No. 5,164,292) of the total silver amount for use in the emulsion grain formation is added, and then subjecting the doped grain to chemical sensitization in the presence of a nucleophilic agent.
It is possible to prepare a photographic emulsion having high sensitivity to a certain degree by the chemical sensitization method using the above-described selenium compound. However, this technique is not yet sufficient to prepare a higher sensitive photographic emulsion or photographic material and a further improvement in techniques has been demanded.
The photographic emulsion prepared using the above-described selenium compound and the photographic material using the same are usually inferior in the incubation durability, and therefore an improvement in this point has also been demanded.
The present inventors have made investigations on a method for achieving high sensitivity using a selenium compound as disclosed in the above-described publications. As a result, an emulsion having high sensitivity as compared with that obtained by gold-sulfur sensitization which has hitherto been employed and a photographic material using the same have been obtained. However, further investigations and improvements are needed to achieve sensitivity on an intended level without deteriorating the incubation durability.